Though it is not the infallible oracle I once thought it was, I still make frequent use of

% perl -MO=Deparse,-p -e '<Perl code>'
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...to figure out how perl is parsing something, or maybe to get a grip on some obfu. E.g. if you wonder what this one-liner does

-nle '}{print$.' foobar.txt
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feed it to -MO=Deparse,-p (make sure to retain any other command line switches, like -nl here):

% perl -MO=Deparse,-p -nle '}{print$.'
BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined(($_ = <ARGV>))) {
    chomp($_);
}
{
    print($.);
}
-e syntax OK
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Note that the -p in this one-liner is part not perl's command line switch, but rather an argument to B::Deparse.

Less commonly , I use other backends, such as B::Terse and B::Concise, instead of B::Deparse. See also O and B.

the lowliest monk